Natural gas is typically considered “sour” if the hydrogen sulfide content exceeds 5.7 milligrams of H2S per cubic meter of natural gas. The process for removing hydrogen sulfide from sour gas is called gas “sweetening.”
The drawbacks of presence of H2S in gas streams are well known. These drawbacks are the toxicity of hydrogen sulfide to humans and animals on the one hand and their high corrosive effects, which damage the process apparatus, on the other hand. The H2S concentration in gas streams should therefore not exceed 4 ppm.
Due to its toxicity, hydrogen sulfide gas cannot be vented into the atmosphere, neither can it be burnt; its combustion produces another highly poisonous and corrosive species, namely SO2. Thus, it is preferred that hydrogen sulfide is eliminated from the gas streams by other chemical means.
Different processes are known to achieve this aim. According to one of these processes, aqueous solutions of metal chelates, especially of iron(II) chelates, are used. The solutions of iron(II) chelates must have an alkaline pH-value. During the sweetening process, H2S reacts with the iron(II) in the chelate which is subsequently reduced to iron(II) and H2S is directly converted to elemental sulfur. This process is usually carried out in an absorption tower. In order to regenerate the iron(II) species, the mixture is then contacted with oxygen containing gases (such as air) in a regeneration tower, so that the solution can be recycled.
Common chelating agents used to prepare the mentioned iron chelates include ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and hydroxyethyl ethylenediaminetriacetic acid (HEDTA). All of these chelating agents are usually present as sodium salts. The product of the reaction of hydrogen sulfide with these solutions comprising iron(III) chelates is solid sulfur. If the solid sulfur particles are too small, they are difficult to be separated from the solution, which will lead to plugging of pipes and pumps. This will also increase the production of unwanted sulfur species like SOx, thereby reducing the overall sulfur removal efficiency and, unfortunately, also increasing the amount of poisonous waste.
The presence of chelating agents such as EDTA, especially when not in complex with iron, will significantly increase the corrosive activity of the sweetening solution towards the alloys of the sweetening system. This is especially true for sweetening systems that are at least in part made of carbon steel.
In the past, many efforts have been made in order to overcome the problems of the very high corrosion rates of redox solutions on carbon steel alloys, on the one hand, and sulfur particles that are too small to sediment efficiently, on the other hand. These small suspended particles block the pumps and tubes, thereby reducing the circulation rate of the catalyst and hence decreasing the overall process efficiency. This effect further leads to the formation of sulfate and thiosulfate species that are produced due to the inevitable contact of the particles with oxygen in the air.
There are many sweetening solutions disclosed in the prior art that are also based on iron ions complexed by chelating agents like ethylenediaminetetraacetic acid (see for example European patent application 1 656 992). The sweetening solutions known from the art suffer the drawback of high corrosion effects towards the tubes and pumps of the sweetening system and lead to very small sulfur particles that are difficult to separate from the stream.
U.S. Pat. No. 4,515,764 discusses problems of small sulfur particles. The drawback of the sweetening solution described in this document is the thiosulfate present in the composition, leading to very high corrosion rates.
Document U.S. Pat. No. 4,356,155 discloses a process, in which a solution of nitrilotriacetic acid chelates is used for the complexion of the iron. The solution further contains alcohols of the general formula CnH2n-1OH, wherein n=4 to 22. This composition addresses the problem of small sulfur particles, while the high corrosion rates are not addressed. The same applies to document U.S. Pat. No. 4,909,945, wherein polymeric coagulants are used to increase the particle size of the produced sulfur. This effect is achieved by sequentially injecting the coagulant to the process solution. Again the drawback of this process is the high corrosive activity of the solution.
The drawback of high corrosive activity is addressed by document U.S. Pat. No. 5,616,306, wherein high contents of phosphate are applied in a hydrogen sulfide removal process. This process suffers the disadvantage of leading to only a small decrease in corrosion rate, while not having any influence on the sulfur particle size.
It can easily be seen from the state of the art that there is a strong need for gas sweetening solutions capable of producing large sulfur particles and simultaneously reducing the corrosive activity of the solution significantly.